Fuel cells are promising devices for clean power generation in a variety of economically and environmentally significant applications. By using hydrogen produced from renewable energy sources, such as solar and wind, fuel cells can provide carbon-neutral power without any pollutants, such as SOx and NOx. Initial commercialization of clean, high-efficiency fuel cell electric vehicles is already underway, but further technological innovation is needed to improve cost-competitiveness of fuel cells in the marketplace.
Currently, there are two general types of fuel cells: low temperature fuel cells and high temperature fuel cells. Low-temperature proton exchange membrane (PEM) fuel cells utilizing Nafion® polymeric materials for membranes require a high level of hydration, which limits the operating temperature to less than 100° C. to preclude excessive water evaporation. The structure for Nafion® is provided below.
Low-temperature PEM fuel cells that use Nafion® are currently being commercialized in fuel cell vehicles, but these cells can operate only at relatively low temperatures and high hydration levels (FIG. 1A); therefore, they require humidified inlet streams and large radiators to dissipate waste heat.
In contrast, high-temperature PEM fuel cells typically utilize membranes comprising phosphoric acid-doped polybenzimidazole, shown below.
High temperature fuel cells can operate effectively up to 180° C.; however, these devices degrade when exposed to water below 140° C. High-temperature PEM fuel cells that use phosphoric acid (PA)-doped polybenzimidazole (PBI) could address these issues, but these PBI-based cells are difficult to operate below 140° C. without excessive loss of PA (FIG. 1B). The limited operating temperature range makes them unsuitable for automotive applications, where water condensation from frequent cold start-ups and oxygen reduction reaction at the fuel cell cathode occur during normal vehicle drive cycles.
Quaternary ammonium functionalized polymers are known, and some have been developed for alkaline electrochemical devices. As currently understood, phosphoric acid-doped QA functionalized polymers have been reported only once, by the Wegner research group at Max Planck Institute in 1999, A. Bozkurt et al., Proton-conducting Polymer Electrolytes based on Phosphoric Acid, Solid State Ionics, 125, 225 (1999). Bozkurt et al. used poly(diallyldimethylammonium) as the polymeric material used to produce the fuel cell membrane, and their approach was substantially the same as that of the PA-doped PBI in three respects: 1) the quaternary ammonium moiety of the synthesized polymer was located within the polymer backbone; 2) the quaternary ammonium moiety concentration was high (about 7.2 mmol/gram, which is comparable to that of PBI, about 6.5 mmol/gram); and 3) the researchers were primarily interested in anhydrous proton conductivity.
Despite the substantial development of fuel cell technology, there still is a need for developing fuel cells that address the substantial limitations associated with both low temperature and high temperature fuel cells.